The housing or the encapsulation of micromechanical systems (Microsystems) may be done both for individual devices and for a wafer, which may comprise a multiplicity of devices. Micromechanical systems often have a moveable mechanical structure, and the present invention concerns an encapsulation of microsystems, using wafers, and particularly such systems where a relatively large cavity is necessary and/or desirable to guarantee flawless functioning by free movement of the micromechanical structure.
So as to ensure long life of the micromechanical systems or of MEMS (micro electromechanical systems) devices, moveable parts need suitable protection by encapsulation or housing. The encapsulation of microsystems is a critical part of the packaging process, which is traditionally performed only after dicing of the devices by a so-called die-by-die process. Encapsulation using wafers typically having a multiplicity of devices opens up the possibility of substantial cost reduction, since particular handling of individual devices is avoided. The encapsulation may be done immediately after the MEMS is finished or enabled, which at the same time offers protection for the device during the ensuing procedural steps, particularly the dicing of the devices. Thus, as a result, simple processing is obtained, and increased overall yield may be achieved.
In the past few years, a series of solutions have been presented, such as in K. Najafi, “Micropackaging technologies for integrated microsystems: applications to MEMS and MOEMS”, Proc. SPIE, vol. 4979, 2003. In M. B. Cohn, et al., “MEMS packaging on a budget (fiscal and thermal)”, IEEE Conference on Electronics, Circuits and Systems, 2002 and in W. Kim et al., “A low temperature, hermetic wafer level packaging method for RF MEMS switch”, IEEE Electronic Components and Technology Conference 2005, hermetic packages using thermal-pressure bonding by means of metal sealing are described; so-called anodic bonds are described in V. Kaajakari, et al., “Stability of wafer level vacuum encapsulated silicon resonators”, 2nd International Workshop on Wafer Bonding for MEMS Technologies, Halle/Saale Germany Apr. 9-11, 2006. Bonds utilizing localized heating of metals are presented in L. Lin, “MEMS post-packaging by localized heating and bonding”, IEEE Transactions on advanced packaging, vol. 23, no. 4, November 2000, and so-called glass frit bonds in D. Sparks, et al., “Reliable vacuum packaging using NonoGetters™ and glass frit bonding”, Proc. SPIE, vol. 5343, 2004. Most of these known solutions necessitate so-called wafer bonding. Alternative solutions, however, are also known, such as the formation of cavities by thermal decomposition of special polymers, see P. Monajemi et al., “A low cost wafer-level MEMS packaging technology”, IEEE MEMS 2005.
MEMS devices generally are sensitive with respect to air humidity, which easily develops in changing air conditions and may lead to corrosion and/or static friction. For this reason, encapsulation and/or housing usually is necessary for reliable functioning. Due to the high permeability or penetration of the air humidity, polymer packagings generally are avoided. For really hermetic packagings, metals or glass are used as materials for the housing or the encapsulations as well as for the sealings. Hermetic encapsulations generally are very costly and for example represent 50-80% of the costs for the MEMS devices, see B. Cohn, et al., “MEMS packaging on a budget (fiscal and thermal)”, IEEE Conference on Electronics, Circuits and Systems, 2002. Some MEMS devices/systems, such as ones with large mechanical structures or micromachines, are less sensitive with respect to air humidity and therefore do not need expensive hermetic encapsulation. In such applications, a housing, a cover or an encapsulation of polymer material provides sufficient protection during the process of dicing and packaging. Such a solution is an encapsulation structure, such as described in Y.-M. J. Chiang et al., “A wafer-level micro cap array to enable high-yield micro system packaging”, IEEE Transactions on Advanced Packaging, vol. 27, no. 3, August 2004, for example, which allows for mass production in great numbers at low costs. The packaging described here is done by means of a mold and is disadvantageous in that high costs for the mold result. A further disadvantage is the optical window for the polymer material used, which has lower optical quality than when using glass, for example. For example, this is a result of the different absorption behavior of glass as compared with a polymer material.
It is a particular challenge to form housings or encapsulations having a large cavity for a MEMS device, so that movement in the range of 10 to 300 μm and more outside a plane is possible. The formation of so-called spacer frames (i.e. layered structures functioning as spacers) in silicon, for example, by potassium hydroxide etching (KOH etching) is possible and/or known, see DE 199 40 512. Structures thus obtained, however, are expensive and inflexible.